Fiber optic communications requires at minimum a transmitter, a propagation medium, and a receiver. The transmitter sends a lightwave signal, containing the data that requires transmission, down the medium. The medium itself consists of optical fiber, as well as optical filters, amplifiers, attenuators, and other devices. The receiver at the far end of the medium converts the optical signal into an electrical signal suitable for devices with which it will interface (e.g., routers, switches, etc).
The most common optical transmission format for digital data in long haul optical networks consists of binary intensity modulation. In this format, a logical “1” corresponds to a pulse of light, while a logical “0” corresponds to the absence of a pulse. The pulses are sent sequentially, at a pre-determined bit-rate. The bit period, or time duration between two consecutive “1” pulses, must be equal to or larger than the pulse width.
Due to dispersion in the medium, pulses tend to broaden over propagation. Without dispersion compensation, the pulse width (in units of time) would lengthen to the point where two consecutive pulses start to interfere with each other. In other words, the pulse-duration grows longer than the bit-period. Chirping a pulse is one known technique for controlling dispersion of an optical signal.
“Chirping” a pulse is inducing a phase modulation in parallel with the intensity modulation. FIG. 1 illustrates the light source frequency, modulated by a pulse shape at a lower frequency, at 2. The envelope shown at 4 is what is “intensity modulated”, and represents the variation of the optical power in time. The additional phase-modulation or chirp is shown at 6. This phase delta would be added to the periodic waveform shown at 2. In the illustration, the phase modulation is shown to be ideally synchronized with the intensity modulation (peaks of 6 align with peaks of 4) for positive chirping. Negative chirping would require aligning the peaks of 4 with the valleys of 6. Shifting the chirp to the left or right would constitute non-ideal synchronization.
FIG. 2 illustrates the broadening of the pulse width as a function of dispersion and chirping. In the case of a pre-chirped pulse (i.e., chirped at the transmitter), the broadening of the pulse is modified. With the appropriate sign of chirp, the pulse will actually start to compress, reach a minimum width, and then broaden again as it traverses through the network. This effect can reduce the amount of dispersion compensation required. The amplitude of the phase modulation governs how much the bit will be compressed, while the appropriate synchronization depends on the net sign of dispersion throughout the propagation medium as is well known in the art.
Pre-chirp is often applied at a fixed amplitude: either by external, finite-chirp, data modulators (chirp is intrinsic to the intensity modulation), by direct-modulated lasers, or by external phase modulators. Typically, a fixed amplitude of pre-chirp is determined at design time, such that it will cover a large range of dispersion values. To maintain the amplitude, a peak detector can be used, which assumes fixed characteristics of the optical transfer function over lifetime and temperature. However, the performance of the communication link could be improved by optimizing the chirp for transient network conditions.
When pre-chirp is applied with an external phase-modulator, it is very easy to adjust the amplitude of the chirp. Therefore, it is desirable to provide an adaptive method for determining and controlling the ideal chirp amplitude applied to an optical signal.